This invention relates to drive circuits for fluorescent lamps. More particularly, this invention relates to a fluorescent-lamp power-supply circuit that uses piezoelectric characteristics of certain ceramic materials to produce a high-frequency, high-voltage excitation output to drive the lamp.
Fluorescent lamps are finding increased use in systems requiring an efficient and broad-area source of visible light. For example, portable computers, such as lap-top and notebook computers, use fluorescent lamps to back-light or side-light liquid crystal displays to improve the contrast or brightness of the display. Fluorescent lamps have also been used to illuminate automobile dashboards and are being considered for use with battery-driven, emergency-exit lighting systems in commercial buildings.
Fluorescent lamps find use in these and other low-voltage applications because they are more efficient, and emit light over a broader area, than incandescent lamps. Particularly in applications requiring long battery life, Such as in the case of portable computers, the increased efficiency of fluorescent lamps translates into extended battery life, reduced battery weight, or both.
In low-voltage applications such as those discussed above, a power-supply and control circuit must be used to operate the fluorescent lamp. While power typically is provided by a DC source ranging from 3 to 20 volts, fluorescent lamps generally require AC voltage sources of about 1000 volts RMS to start, and over about 200 volts RMS to maintain illumination efficiently. Excitation frequencies for fluorescent lamps typically range from about 20 kHz to about 100 kHz. Accordingly, a power-supply circuit is needed to convert the available low-voltage DC input into the needed high-frequency, high-voltage AC output.
Previously known fluorescent-lamp power-supply circuits have used various drive circuits that convert the low-voltage DC into high-frequency, low-voltage AC having a waveform that approximates a sine wave. The low-voltage AC was then applied to the primary coil of a magnetic step-up transformer. The secondary coils of the step-up transformer produced the high-frequency, high-voltage AC needed to start and maintain illumination of the fluorescent lamp. The output voltage of the step-up transformer is related to the input voltage by the well known relationship: N2/N1=V2/V1, where N1 is the number of turns on the primary coil, N2 is the number of turns on the secondary coil, V1 is the input voltage, and V2 is the output voltage. The ratio N2/N1 is commonly known as the "turns ratio."
The use of magnetic step-up transformers to supply high voltages to applications described above is widespread and well known. Such transformers employ common magnetic techniques to achieve the low-to-high voltage conversion. However, the use of magnetic step-up transformers in power-supply circuits for computer back-lights creates certain undesirable characteristics.
Some key disadvantages of previously known fluorescent-lamp power-supply and control circuits are related to the inherent geometric limitations posed by using a magnetic step-up transformer on the ability to minimize the size of the power-supply circuits.
For example, a long-standing goal in a typical lap-top computer system has been to maximize display area on the "clam-shell" (or lid which contains the display) of the computer while minimizing exterior dimensions of the computer, and thus the size of the power-supply circuits driving the computer's display. It has long been accepted that the power-supply circuit for the display must reside in the clam-shell itself and not in the base (typically containing, among main circuitry, the hard and floppy drives, the keyboard, the battery, etc.). The reason is that driving the back-light requires a high-frequency, high-voltage AC signal that is subject to loss or noise over any extended line. Driving the display in the clam-shell with a power-supply circuit in the base would result in significant losses and thus would not work adequately.
The geometric nature of magnetic step-up transformers and the sheer number of turns required to obtain desired input/output characteristics for driving a fluorescent back-light display limit the ability to make prior art power-supply circuits smaller. A higher output voltage requires more turns, which in turn increases the size of the magnetic transformer.
Another disadvantage of lamp-driving power-supply circuits using magnetic step-up transformers is that failure of the magnetic transformer can be catastrophic to both itself and surrounding components. Because of the high voltages involved in driving fluorescent lamps and the inability of magnetic transformers to handle high-voltage failure, a short-circuit in the output terminals of a magnetic step-up transformer may result in extreme heat dissipation from the transformer towards surrounding electronics. A short-circuit between two turns in the secondary coil could have the same effect. Thus, a short-circuit may generate heat which could damage or destroy the display in addition to damaging the driving electronics.
Furthermore, short-circuits between two turns in a magnetic transformer can be difficult to prevent. First, a high number of secondary turns is required to produce the desired output voltage. Second, the turns are compressed into the most compact package possible to minimize the exterior dimensions of the magnetic device. Therefore, the insulation between each turn is minimal and subject to deterioration.
Using a magnetic transformer presents yet another disadvantage in the event that the fluorescent lamp breaks. A broken lamp basically results in an open-circuit or infinite resistance output for the power-supply circuit. When the circuit detects such a high load, it attempts to drive the load at an even higher voltage (for example, 2-3 times the normal voltage), leading to potentially dangerous high-voltage conditions. A subsequent short-circuit at the output or between turns in the transformer can cause catastrophic failure not only to the transformer itself, but also to the surrounding circuitry. Such failures can present or induce hazards not limited to the computer if the computer is used in a sensitive environment, such as a lap-top being used on an airplane.
The nature of the magnetic transformer as a broadband device presents yet a further disadvantage for its use in power-supply circuits for driving fluorescent lamps. Because the fluorescent lamp requires a sinusoidal (AC) input signal, the power-supply circuit must produce a sinusoidal output--if no conversion takes place between the output stage of the power-supply circuit and the input stage of the fluorescent lamp. A magnetic transformer, like other broadband devices, requires a sinusoidal input to produce a sinusoidal output. Thus, the magnetic transformer cannot directly accept a square-wave input. A square-wave signal has to be converted to a sinusoidal signal before being applied to the magnetic transformer in order for the transformer to provide a sinusoidal output. This required conversion process adds complexity to the design of the power-supply circuit.
In view of the foregoing discussion, it would therefore be desirable to provide a power-supply and control circuit for a fluorescent lamp that uses a device with inherent fundamental characteristics that are more conducive to the device being used as a step-up transformer than the characteristics of a conventional magnetic-coil transformer.
It would also be desirable to provide a power-supply and control circuit for a fluorescent lamp using a step-up transformer that can be inherently made smaller in all dimensions than conventional magnetic transformers.
It would further be desirable to provide a power-supply and control circuit for a fluorescent lamp that can be made smaller than conventional circuits using magnetic step-up transformers.
It would additionally be desirable to provide a power-supply and control circuit for a fluorescent lamp having a simpler construction than conventional, magnetic, power-supply circuits.
It would further be desirable to provide a power-supply and control circuit for a fluorescent lamp that allows relatively safer high-voltage failure in the step-up transformer, so as not to dissipate extreme heat or induce failure in the electronics surrounding the transformer.
It would still further be desirable to be able to provide such a fluorescent-lamp power-supply and control circuit that uses a step-up transformer with a relatively high internal resistance and ability to withstand high-voltage operation.
It would still further be desirable to provide a fluorescent-lamp power-supply and control circuit using a transformer that does not require a sinusoidal input in order to generate a high-frequency, high-voltage AC output.